1. Field of the Invention
The present invention relates to a method for producing a photomask and, more particularly, it relates to a method for easily producing a photomask having good quality using a silver halide photographic material.
2. Description of the Prior Art
It is known in the field of printed circuit fabrication or microelectronic fabrication to use a photomask in the case of forming images utilizing a photoresist method. Heretofore, photomasks have often utilized silver images or chromium images. Silver images are obtained by imagewise exposing a photographic light-sensitive material (a so-called emulsion mask) provided on a glass plate, followed by ordinary photographic processings (e.g., development and fixing). On the other hand, chromium images are formed by providing a vacuum-deposited chromium layer on a glass support and etching the chromium layer using the photo etching method. That is, a photoresist is coated on the chromium layer and, after exposing through a photomask superposed thereon (contact method) or after projecting a photomask on the photoresist (projection method), the photoresist is development-processed to form a resist image, followed by etching the chromium layer.
Since an emulsion mask has high sensitivity, it is suitable for preparing a photomask. However, the resolving power of a photo mask obtained is inferior to that of a chromium mask using the projection method due to scattering of light by the silver grains, the high thickness of the silver halide emulsion layer and the silver particles after development. A photomask utilizing silver images is inferior in resolving power for the following additional reason. That is, since the developed emulsion thickness is larger at silver image areas than at non-image areas, gaps or spaces are formed at the contact surfaces upon intimately contacting the photomask with the photoresist layer in order to form a resist image, thus markedly reducing the resolving power. In addition, a photomask formed using a silver image has such a small mechanical strength that it is easily damaged. Also, since the silver image does not transmit visible light, positioning of the mask is difficult.
On the other hand, a chromium mask using a contact method is inferior to a photomask obtained with a silver image in resolving power, since the chromium mask suffers reduction in resolving power upon exposing a photoresist layer and upon etching the vacuum-deposited chromium layer. Also, with a chromium mask using a projection method, the photoresist layer has such a low sensitivity that exposure requires too much time, thus it is difficult to practice industrially.